Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft either directly drives the generator rotor (“directly driven”) or through the use of a gearbox.
An important trend in the field of wind turbines is to place the turbines in offshore wind parks. These wind parks comprise a plurality of wind turbines and a local wind park grid. This wind park grid may be connected to an onshore electrical grid through a High Voltage Alternating Current (HVAC) transmission.
In known HVAC transmission systems voltage instability problems may occur which may cause a voltage collapse in case of severe disturbances, like deep voltage sags. Voltage stability may be defined as the ability of a power system to maintain steady acceptable voltages at all buses in the system under normal operating conditions and after being subjected to a disturbance.
A system may enter a state of voltage instability when a disturbance, an increase in load demand, or a change in system conditions, cause a progressive and uncontrollable drop in voltage. An important factor causing instability is the inability of the power system to meet the demand for reactive power. At the heart of the problem is usually the voltage drop that occurs when active and reactive power flow through inductive reactances associated with the transmission network.
Voltage instability is a local phenomenon; however, its consequences may have a widespread impact. Voltage Collapse is more complex than simple voltage instability and is usually the result of a sequence of events accompanying voltage instability leading to a low-voltage profile in a significant part of the power system. Solutions that only take into account local voltages may not be good enough to predict and/or prevent voltage collapse for electrical grids involving offshore wind parks.
The prediction of voltage instabilities with enough anticipation in order to avoid damaging consequences is fundamental. In this respect, it is known to use a Voltage Collapse Prediction Index (VCPI) consisting of calculating this index at every bus of the power system in offshore wind parks. The calculation of this VCPI requires to have voltage phasor information of the buses in the system and to know the network admittance matrix. The value of VCPI may determine the proximity to voltage collapse at a specific bus. A disadvantage is that the index used is rather complex and the information necessary to use it may not even be available to wind park operators.
In this respect, a simpler index that is able to assist in avoiding voltage collapse is desired.